1. Field of the Invention
The present invention relates to optical waveguides. More particularly, it relates to rotationally non-symmetric optical waveguide fibers well-suited for lateral coupling and to methods for forming such fibers.
2. Prior Art
Optical waveguide fibers typically include an optical energy transmitting core having a selected index of refraction surrounded by a cladding with an index of refraction lower than that of the core. Methods for forming optical fibers typically involve fabricating a multipart preform in which the interior or core part has an index of refraction greater than that of a surrounding outer portion or cylindrical tube. The core preform can be fabricated, for example, by building a doped solid silica cylinder, usually by vapor deposition, to increase the index of refraction above that of undoped silica. Regardless of the manner by which the preform is fabricated, the preform is heated to a temperature greater than its drawing temperature and drawn down in one or more drawing stages to form the resulting optical fiber.
Optical fibers may be coupled to one another either by butt coupling or by lateral coupling. With butt coupling, the ends of the fibers are cut or otherwise formed with an end surface transverse to the longitudinal axis of the fibers and the fibers supported in an end-to-end relationship. Alignment of the butt coupled fibers must be precise to achieve necessary axial core-to-core registration. The alignment problem can be particularly critical with single mode fibers in which core diameters range between 1 and 5 microns. With lateral coupling, portions of the fibers to be joined are placed in overlapping side-by-side relationship with energy transfer effected through evanescent field coupling between the laterally adjacent cores.
The techniques of lateral coupling have been greatly facilitated by the development of optical fibers, such as those disclosed in U.S. Pat. No. 4,315,666 which are rotationally non-symmetric along their length, for example, by placement of the core eccentrically in an otherwise circular fiber. Two such optical fibers can be aligned in a side-by-side relationship and rotated relative to one another to, in effect, "tune" the lateral coupling for optimum optical energy transfer.
In addition to optical fibers constructed as described above, other types of optical fibers have been manufactured by designing core and cladding structures that place the core under a stress relative to the cladding to cause stress-induced birefringence and resulting polarization locking on mutually orthogonal transverse axes. One such stressed-core optical fiber structure and method for fabricating the fiber is disclosed in U.S. Pat. No. 4,354,736 to Maklad et al. As disclosed therein, a circularly symmetric optical energy transmitting core is placed in contact with and supported on two sides within a hollow, elliptical substrate tube or jacket so that the core is at least partially surrounded by an air cladding. The elliptical substrate tube places the core under a compressive stress along one axis to cause a desired stress-induced birefringence. Optical fibers utilizing core and cladding structures that place the core under a stress to achieve birefringent light transmitting characteristics generally position the energy transmitting core within a structure that makes lateral coupling less convenient than with rotationally non-symmetric optical fibers of the type described above and as disclosed in the aforementioned U.S. Pat. No. 4,315,666.